Hi @Dan28 I am sorry that you are having to deal with this. I know full well the price of this delicacy as I've had sticker shock after ordering it before.
I'm a host with 6+ years of experience and this is what I would do - I would take the loss. It's a lousy situation, but if you didn't have the item more securely put away (of course I realize you wouldn't put it in a safe) or if you hadn't left prior instructions for the guest on what was/was not available to them, the guest could have easily assumed it was available to them to use, just as an open jar of sundried tomatoes might be. Then when confronted they might have been absolutely mortified to discover they'd eaten something so precious, and rather than admit it, they denied it. This is a different situation then someone breaking into a locked cabinet and taking something.
Honestly take it as a loss, and move on. Luckily as a chef you'll have access to buy more and keep the receipt for the replacement and use it as a business expense on your tax return.
I've stupidly left a bottle of Grand Cru in my wine cabinet before and totally forgotten to take it out before guests arrived, and guess what? The young French men that arrived were delighted to have it and even commented to me that it was so nice to find a selection of lovely wines. A big "oh **bleep**" moment for me.